An integrated circuit device may be formed by depositing one or more conductive layers having a predetermined pattern and one or more insulating layers on a substrate. For example, transistors having a gate, a source region, and a drain region may be formed in active regions of the substrate, and the source/drain regions may be electrically connected to an upper conductive layer, such as a bit line or a lower electrode of a capacitor, through a contact structure.
A method of forming a conventional self-aligned contact structure will be described with reference to FIG. 1. In FIG. 1, a gate pattern 16, which comprises a gate insulating layer 12, a gate electrode 13, a capping layer 14, a pair of spacers 15, and a source/drain region 11 are formed on a substrate 10. An interlayer insulating layer 17 is then formed, which covers the gate pattern 16. Next, the interlayer insulating layer 17 is etched in a self-aligned manner to form a contact hole 18. The contact hole 18 is filled with a conductive material to form a contact pad 19, and then an upper conductive layer (not shown) having a predetermined pattern is formed on the contact pad 19 and the interlayer insulating layer 17 to form a self-aligned substrate contact structure. The capping layer 14 and the spacers 15 may protect the gate electrode 13 when the interlayer insulating layer 17 is etched to form the contact hole 18.
The self-alignment process may enable formation of a contact in increasingly more highly integrated devices, which may use a narrow area for the contact, and may also ensure a sufficient alignment margin for a photolithographic process. As the integration density of integrated circuit devices continues to increase, however, contact resistance may be problematic. As integration density increases, contact resistance generally increases due to the resistance of polycrystalline silicon and the decreased contact area between a contact pad and a source/drain region. The increase in contact resistance may lower the operational speed and/or hinder the integration of an integrated circuit device.
Several techniques have been used to reduce contact resistance, including: 1) the concentration of impurities used in a process for doping polycrystalline silicon may be increased, 2) the contact pad maybe formed of metal, and/or 3) the contact area between a contact pad and a source/drain region may be increased. The first and second techniques may result in impurities or metal diffusing into a source/drain region so that the electrical property of a device may be changed. The third technique may involve increasing the area of a contact hole for the purpose of increasing the contact surface area between a contact pad and a source/drain region. This approach, however, may not be compatible with increasing integration density.
FIGS. 2A and 2B illustrate integrated circuit devices comprising a substrate, 20, a device isolation layer 21, a gate insulating layer 23, a gate electrode 24, an interlayer insulating layer 25, and a contact hole 26. Silicon layers 27 and 27′ having the same conductivity type as a source/drain region 22 are formed in a particular area exposed by a contact hole 26 belonging to the source/drain region 22, thereby increasing the contact area between an upper conductive layer 28 and the source/drain region 22. Unfortunately, the upper conductive layer 28 is in direct contact with the source/drain region 22 at the edge of each of the silicon layers 27 and 27′. As a result, impurities or metal from the upper conductive layer 28 may diffuse into the source/drain region 22.